Hydraulic positioners having a high weight capacity are well known for manufacturing operations, such as manual welding and manual assembly. For example, applicant's prior art positioners can elevate and rotate the workpiece at either the head or tail end, or both, with programmed work points having repeatability within +/−0.250 inch for each axis, thereby leading to a maximum repeatability variation of +/−0.750 inch relative to the programmed work point for the elevating headstock and tailstock with a rotary axis. This variability of the repeatability range is undesirable for automated procedures, such as robotic welding. Although these types of positioners are lower cost than traditional servo robotic positioners programmed point position variability limits their use for robotic manufacturing applications.
Applicant's prior art positioner utilizes a single acting hydraulic cylinder for each of the headstock and tailstock carriages. Starting from a “home” loading position, the workpiece is attached to a workpiece holding fixture mounted on the headstock and tailstock, which are then raised or lowered independently while maintaining a substantially horizontal plane between the opposite workpiece ends. Movement of the workpiece continues until the pre-programmed work position is achieved. A cable-actuated string potentiometer is used to measure linear position of the headstock and tailstock carriages and provides the PLC or control with the required programmed position information. After the workpiece is elevated, gravity provides the forced return of the headstock and the tailstock to the home position. Each headstock and tailstock carriage include a plurality of teeth as a safety precaution, in the event of hydraulic cylinder failure. If a cylinder failure or blowout occurs during positioner elevation or descent, a hinge plate mounted on the structures vertical column engages the horizontal surface of the carriages safety tooth immediately above the hinge plate.
The normal open position of the hinge plate is approximately 30 degrees from vertical and is pushed out of the way by the slopping tooth surfaces during elevation. During programmed descent a solenoid is actuated overcoming gravity and spring pressure of the normally open hinge position hinging the plate into a flat position against the structural columns internal surface. This allows gravity to safely descend the carriage and attached fixture/workpiece load to a lower programmed position.
The speed of the prior art positioner assent and descent is controlled and adjusted by the hydraulic cylinder pump on each headstock and tailstock via low and high flow valves. The fluid flow adjustment response to the position and speed information is provided by the string potentiometer. Smooth, controlled movement of the workpiece is preferred, even with unbalanced weight distribution. However, the choice of only a high or low flow rate can produce a halting, stepping motion. Since the high and low flow valves are either completely on or completely off, the motion control can be course or rough, and programmed can position variability can be +/−¼ inch. In addition, over time, the string potentiometer used for position information can lose tension and become constricted due to the manufacturing environment conditions which may undermine position precision.
The current prior art positioners of Applicant also use a hydraulically powered slew worm drive gearbox with a single or multidirectional proportional control valve to rotate each headstock or tailstock about the center axis. An encoder mounted on the worm drive shaft provides position and speed information to the PLC control system. The encoder mounted on the worm drive shaft is old. The new encoder is a non-contact absolute encoder type mounted on the rotation faceplate of the worm drive gearbox. Mounting the encoder on the work shaft end increases the range of programmed position repeatability, since such location is more remote from the workpiece and includes gear clearance variability, unbalanced loading, and any other mechanical variations up to the work slew drive mounting plate. The rotary programmed position repeatability is +/−0.250 inch.
Robotic welding processes often require positioning assemblies and subassemblies (parts/weldments), particularly for large, heavy parts and fixture combinations weighing 5,000-100,000 lbs. moving through the manufacturing process. Hydraulic positioners typically are not used for such robotic welding applications since these positioners do not return accurately to programmed positions. In robotic welding applications where electric servo motors are utilized, two axes positioning of the large heavy part and fixture combinations is accomplished using servo motor/gearbox sets for each positioning axis. The servo motor/gearbox set is programmable using a PLC or auxiliary axes of the robot to move the workpiece/fixture to a pre-programmed point typically within +/−0.008 inch. The ability of the positioner to return to the preprogrammed point position is called “repeatability.” In addition because of precision repeatability, the servo/gearbox positioners are very expensive. For example, large heavy workpiece positioners typically represent 30-40% of the complete robotic welding system cost, which may total $500,000, or more. This high cost of positioners can make financial justification difficult, thereby reducing automation opportunity & cost savings for industries that manufacture large, heavy workpieces. While the servo motor/gearbox positioning machines have very good programmed position repeatability, the cost of these machines far exceeds the precision needed for GMAW welding operations, where the weld joint variability is much greater than the servo motor/gearbox repeatability.
As an alternative to servo motor/gearbox positioners, hydraulic positioners are significantly less expensive, reducing part positioning costs by approximately half, and cutting overall robotic welding system costs by approximately 30%. However, despite the lower cost, hydraulic positioners generally are not used for robotic welding due to the poor repeatability for the programmed positions. This inaccurate repeatability enhances the chance of robotic collision with the part or fixture as a robot approaches or departs from a weld location. It can also compromise optimal specified weld position requirements undermining weld quality. This repeatability deficiency discourages the use of hydraulic positioners for robotic welding. While improved sensors may help robots find the weld joints, poor hydraulic positioner repeatability increases sensor search time, robot collision risk and diminishes weld quality.
Since conventional hydraulic positioners for large and heavy do not have sufficient accuracy and repeatability, this equipment is seldom used for highly precise, automated procedures, such as robotic welding. Therefore, there is a need for improvements to these positioners so as to allow use in automated or robotic applications. Further, there is a need for more cost-effective positioners for use in automated/robotic applications.
Therefore, a primary objective of the present invention is the provision of an improved hydraulic positioner for heavy and large work pieces which can be used in automated and robotic manufacturing operations.
Another objective of the present invention is a provision of an improved hydraulic positioner for heavy and large work pieces which is economical to manufacture, and durable and safe in use.
A further objective of the present invention the provision of a hydraulic positioner having a hard stop safety and hard stop location feature which is functional at all times.
Another objective of the present invention is a provision of a hydraulic positioner having preprogrammed work points with a minimum variation repeatability range for elevation and rotation.
Still another objective of the present invention is a hydraulic positioner which utilizes absolute encoders for all positioner axes, so as to allow all axes to recover immediately from power outages, without recalibration.
Yet another objective of the present invention is the provision of hydraulic positioner having improved fine motion control and position information, to allow a control system to make subtle position adjustments that assure reliable, controlled landing on a carriage hard stop.
These and other objectives become apparent from the following description of the invention